Since digital techniques have progressed in recent years, in many cases, digital processing is performed for images even in medical fields. Instead of conventional radiography using a film for X-ray diagnosis, a two-dimensional X-ray sensor that outputs an X-ray image as a digital image is being widely used. Digital image processing, such as gradation processing, is becoming more important for the digital image output by the two-dimensional X-ray sensor.
An example of the digital image processing is digital subtraction angiography (hereinafter, referred to as “DSA”). The DSA analyzes X-ray images before and after a contrast agent is injected, by using a computer, and reconfigures only an image of blood vessels with the contrast agent injected. This method does not need a conventional special treatment. Merely by injecting the contrast agent from a vein, a sharp image of blood vessels of the entire body can be obtained. Hence, the method can be performed even for an outpatient. This method is becoming more important for clinical sites. A DSA image that is obtained by the DSA is obtained such that images are acquired before and after the contrast agent is injected into an object, and the image before the contrast agent is injected (hereinafter, referred to as “mask image”) is subtracted from the image after the contrast agent is injected (hereinafter, referred to as “live image”). In the subtraction processing of the mask image from the live image, a blood vessel region, which is a region of interest for the diagnosis, is held as a change region between the images caused by the injection of the contrast agent, and the other unnecessary region is eliminated as a background region to obtain a uniform region. Thus, the generated DSA image is useful for the diagnosis.
Meanwhile, the DSA image has a pixel-value distribution that is significantly different from that of the original live image or mask image. More specifically, image information is concentrated in a very narrow area with respect to a pixel-value area of the entire image, whereas a noise or the like having a small information amount is dispersed in the pixel-value area other than that narrow area. Owing to this, if the gradation processing, like the processing performed for the typical X-ray image to increase the contrast, is applied to the DSA image, a wide gradation range is assigned to the region with the noise or the like having the small information amount, but a gradation range is insufficiently assigned to the useful region for the diagnosis, resulting in the contrast being improper. The gradation processing performed for the DSA image has to be different from the gradation processing performed for the typical X-ray image, and has to take the pixel-value distribution of the DSA image into account.
Conventionally, an X-ray fluoroscopy apparatus has performed gradation processing such as window processing, in which an operator uses an operation panel to adjust a window width and a window level while the operator checks a monitor. Unfortunately, the window processing through the manual operation by the operator is typically a troublesome work. It is difficult to quickly and properly perform the adjustment. In addition, the processing result may vary depending on an operator. To address this problem, widely performed is a method that obtains a feature value through analysis of the subtraction image, which is obtained by subtracting the mask image from the live image, and performs the gradation processing on the basis of the feature value.
Japanese Patent Laid-Open No. 01-017631 discloses a method that obtains image enhanced-characteristic information based on a maximum gradation value and a peak value or a bottom value, which are obtained through histogram analysis performed for a subtraction image, and uses the image enhanced-characteristic information for the window processing. Also, Japanese Patent Laid-Open No. 61-058639 discloses processing that acquires a peak value through histogram analysis, adjusts the peak value to match a predetermined reference value, and hence corrects a base pixel value of a subtraction image.
With these methods, since the feature value of the gradation processing is obtained by analyzing the subtraction image, the gradation processing suitable for the pixel-value distribution unique to the DSA image can be performed. In particular, these methods are effective to automatically adjust the gradation of the entire image to fall within a constant range suitable for the diagnosis.
A wide region of the DSA image is the background region having the uniform pixel value. The impression of the entire image depends on the pixel-value range within which the pixel value of the background range falls. The purpose of use of the DSA image, in view of the diagnosis, is observation of the blood vessel region. Hence, it is desirable to increase visibility of the region with the contrast agent injected.
With the conventionally typically used methods based on the histogram analysis, although a stable image having the gradation of the entire image falling within the constant range can be obtained, the conventional methods do not include an idea for the increase in visibility of the region with the contrast agent, the region which is particularly important for the diagnosis.